The study will assess the effect of bosentan on pulmonary vascular resistance and exercise capacity in Sickle Cell Disease (SCD) patients diagnosed with Pulmonary Hypertension. It consists of 3 phases: screening, treatment and follow-up. During the screening visit, the study doctor will decide if patients meet the study requirements. All potential patients will have a diagnosis of increased pulmonary artery pressures that is shown by right heart catheterization conducted shortly prior to start of study treatment. Patients will be asked to perform exercise capacity test (walking as far as possible for 6 minutes). Following the baseline visit the treatment phase consists of 4 additional clinic visits during which the good and bad effects of the drug are reviewed and exercise capacity test will be repeated. Patients will be treated for 16 weeks. Blood samples will be collected every month, or more often, if needed. At the end of the study some of the patients will be asked to repeat the right heart catheterization. All patients will repeat an exercise capacity test. After completion of the study, patients will have the option of enrolling in a long-term follow-up study where all patients will receive active drug. Patients electing not to participate in the extension study will be followed up for safety assessments for about 28 days after the end of the study treatment.